1. Field of the Invention
The present invention relates to television special effects generators of the type which may combine segments of several incoming video signals into a single output video signal. More particularly, the present invention relates to rotational special effects generators which are able to rotate the line or curve which separates adjacent image segments so as to achieve windshield wiper, propeller, fan, clock-hand, and other rotational special effects.
2. Brief Description of the Prior Art
Special effects generators are used in television studios to take portions of the signals generated by two separate television cameras or other video sources and to combine the portions into a single video signal. When the resultant signal is fed into a television image display device, the device generates a composite image which is composed of segments of the images captured by the two television cameras or sources. Typically, the image segments which comprise the composite image are separated from one another by straight lines which hereinafter are called "lines of separation". For example, a horizontal line of separation may separate an upper image segment derived from a first camera and a lower image segment derived from a second camera. Similarly, a diagonal line of separation may separate an upper triangular image segment from a first camera and a lower triangular image segment from a second camera. The slope of slant of a line of separation may conveniently be defined by the angle which the line makes with the horizontal.
The controls of a special effects generator are typically arranged so that the position of a line of separation may be shifted by moving a lever. A "wipe" is a special effect in which a line of separation moves progressively across the composite image, thus giving the illusion that one image segment is "wiping" another image segment right out of the composite image. Conventional special effect generators are typically able to generate horizontal, vertical, and diagonal wipe effects in which a line of separation moves up, down, or sideways but does not rotate.
The present invention has, as its primary object, the achievement of rotational wipe effects using digital logic circuitry. A rotational special effect is one in which a line of separation is free to rotate about a fixed point.
Patent application Ser. No. 131,300 filed by Nikola B. Tkacenko and assigned to the same assignee as the present application, now U.S. Pat. No. 3,812,286 which issued on May 21, 1974, describes analog circuitry which may be used to generate a limited number of rotational special effects. That application teaches how horizontal and vertical sawtooth waveforms may be mixed in varying quantities by a potentiometer so as to achieve a line of separation between image segments which may be rotated as the potentiometer is manually actuated. The analog circuitry used by Tkacenko is relatively satisfactory, and is entirely suitable for many applications. However, analog circuitry can be difficult to calibrate, since analog circuits for mixing sawtooth waveforms have to include a large number of adjustments. The Tkacenko device has to compare low level analog signals with one another. These low-level-signal comparisons can introduce small amounts of instability into the resultant special effects. When the Tkacenko circuitry is used to generate a "windmill" rotational effect, the center point of the "windmill" may typically be observed to "wobble" around a very small circular path rather than to remain absolutely stationary. It is an object of the present invention to provide a rotational special effects generator which is free from the somewhat undesirable features just listed.
U.S. Pat. No. 3,758,712 filed by John P, Hudson and assigned to the same assignee as the present application discloses a digital special effects generator. The digital generator disclosed in that application is far more stable and far easier to calibrate than are comparable analog special effects generators. Because digital circuits measure time intervals and not voltage levels, digital circuits are not subject to drift and require far fewer calibration adjustments than do comparable analog circuits. The present invention is an improved version of the basic digital special effects generator disclosed in the above mentioned application which is able to generate rotational effects in addition to other digitally controlled special effects. In addition to the increased stability which results from controlling rotational effects digitally, the present invention represents a considerable cost savings over the combination of a non-improved digital special effects generator with a conventional analog rotary special effects generator.
The development of digital logic circuitry for controlling the execution of rotational special effects required more than the construction of a digital version of prior analog arrangements. The analog technique of mixing sawtooth waveforms did not lend itself to digital implementation. A new approach to the problem was needed to provide digital circuitry capable of generating a line of separation whose angle with the horizontal can be varied linearly with the motion of a lever or the advancement of a digital counter.